In a multiprocessor system, including a plurality of bus devices coupled to a memory system via a shared bus, a bus device will issue an operation, such as a read operation, requesting a particular portion of data stored within the memory system. In systems employing "snoopy" bus devices, each of the bus devices coupled to the shared bus will monitor the operation to determine whether or not a more recent (often modified) copy of the requested data is contained within that bus device's cache, thus making that modified copy of the requested data the most recent version. This is often referred to as coherency checking, wherein the system insures that the most recent and valid version of requested data is sent to the requesting device, regardless of whether or not the memory system or one of the bus devices currently holds a copy of the most recent version.
When a bus device sends a bus request over the shared bus, there are often several types of reports sent back to the requesting device. One report informs the requesting device of whether or not a "parity error" was detected by one of the bus devices, or whether or not the requested operation was not properly sampled or decoded by one of the bus devices. Another report informs the requesting device of whether or not the resources being requested are presently busy. Another piece of information sent back to the requesting bus device reports on the coherency of the requesting data, i.e., who owns a valid version of the requested data. A problem with these several pieces of information being sent at the same time is that often other bus devices require additional time to snoop their caches because of internal delays within a particular bus device. For example, a particular bus device desiring to perform a snooping operation may still possess a queue of operations that are to be performed previous to the snooping operation. If one or more bus devices incurs such a delay in performing a snooping operation with respect to a particular bus request, the efficiency and performance of the system becomes degraded.
It is often not necessary to wait for a particular bus device to perform a snoop operation, since it may be determined by the system controller that there is an error in the bus request or the intended receiver of the request is not connected to the shared bus or is unavailable, resulting in the request being discarded. Thus, in such instances, it would be desirous to have a technique for reporting such flow control related status information back to a bus device requesting an operation on the shared bus separate from reporting coherency related information so that valuable buffers or snoop bandwidth of other bus participants is not utilized if the flow control information results in a discarding of the bus request.